RFR: 8252596: [TESTBUG] WebPageShim::paint is not thread-safe

[Arun Joseph]

Issue: Tests using `WebPageShim::paint` fails around 1/10 times.

Fix: Execute `WebPageShim::paint` in the render thread using `PrismInvoker`. Ran tests over 100 times, no failures.

Used the initial unit test in the PR for [JDK-8202990](https://bugs.openjdk.java.net/browse/JDK-8202990).

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Commit messages:
 - 8252596: [TESTBUG] WebPageShim::paint is not thread-safe

Changes: https://git.openjdk.java.net/jfx/pull/327/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=327&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252596
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/327.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/327/head:pull/327

PR: https://git.openjdk.java.net/jfx/pull/327